Chapter 5 - Trigonometric Identities - Section 5.3 Sum and Difference Identities for Cosine - 5.3 Exercises - Page 219: 43

$$\cos(0^\circ-\theta)=\cos\theta$$

$$A=\cos(0^\circ-\theta)$$ Apply the cosine difference identity, which states $$\cos(A-B)=\cos A\cos B+\sin A\sin B$$ we have $$A=\cos0^\circ\cos\theta+\sin0^\theta\sin\theta$$ $$A=1\times\cos\theta+0\times\sin\theta$$ $$A=\cos\theta$$